Method for changing the boundary surface tension of a liquid glass piece and apparatus for performing said method

ABSTRACT

The process according to the invention reduces the effort required to shape a glass piece. In this process a liquid glass piece is provided on a structured supporting surface of an electrically conducting base. A voltage is applied across the liquid glass piece by connecting a voltage source between a contact position on the liquid glass piece and the electrically conducting base. The apparatus for performing the process has an electrically conducting base on which the liquid glass piece is supported and a device for applying a voltage between a contact position on the liquid glass piece and the electrically conducting base.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for changing a boundarysurface tension of a liquid glass piece. It also relates to an apparatusfor performing that method.

[0003] 2. Description of the Related Art

[0004] Currently after a casting process for making them manufacturedoptical lenses and also other curved glass bodies are ground in severalgrinding or working steps, which provide them with a predeterminedradius of curvature. The individual grinding and polishing steps howevercause high manufacturing costs because they are highly time consumingand because of the expensive machine engineering.

[0005] A generic process is disclosed in U.S. Pat. No. 3,535,100, inwhich the shape of a liquid glass piece is changed. In this process theliquid glass piece is supported on an electrically conducting supportand an electrical potential is applied to the liquid glass piece bycontacts connectable with the liquid glass piece.

[0006] The electrically conducting support itself is an electricallyconducting liquid in the case of this reference. Current flow within theliquid glass piece changes the boundary surface tension, which changesthe shape of the glass sample. Problems are encountered using thisprocess for optical glass, which has high specifications in regard toshaping and purity. The reason for this is the inhomogeneous behavior ofions contained in the liquid glass piece, especially alkali ions, withprogressing time, since the liquid glass piece cools over the course oftime.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide a genericprocess for changing the boundary surface tension of the liquid glasspiece, whereby the above-described problem due to the inhomogeneousbehavior of ions in the glass piece can be solved.

[0008] It is another object of the present invention to provide anapparatus for performing the process according to the invention.

[0009] The process according to the invention for changing the boundarysurface tension of a liquid glass piece placed on an electricallyconducting base includes applying an electrical potential to the liquidglass piece resting on the electrically conducting base at a contactposition on the liquid glass piece and heating the liquid glass piece onthe electrically conducting base. By heating the liquid glass piece thetemperature of the glass piece is kept constant during the applicationof the voltage to the glass piece.

[0010] The surface tension of the glass in the liquid state isinfluenced by its chemical composition so that, for example, adecreasing content of alkali ions or alkaline earth ions causes anincrease of the boundary surface tension.

[0011] By applying an electrical potential the moving ions, such as e.g.the alkali ion Na⁺, are forced from the boundary surface of the glasspiece into its interior. They are enriched on the negatively polarizedside of the boundary surface and thinned out on the positively polarizedside, which produces an effect on the boundary surface. A positivepolarization of the supporting surface causes an increase of theboundary surface voltage, an increase of the wetting angle andconsequently a lowering of the wetting of the surface.

[0012] With the help of the new method the filling of a certain shapingregion of the supporting surface during shaping can be controlled. Thisis true both in the microscopic sense for microstructures and porosityof the shaping material but also in the macroscopic sense, for examplefor the filling of corners in the shape.

[0013] Because of the reduction of the wetting that the method producesa smoothing of the surface of the liquid glass piece to be shapedoccurs, whereby the roughness of the shaping tool or the local damageare not pressed into the glass body to their full extent. Moreover it ispossible to provide an exact transfer of the structures formed on theshaping tool to the liquid glass piece to be shaped. This would bepossible without the application of voltage when the glass piece is at ahigher temperature, since then both the viscosity and also the surfacetension of the glass piece will decrease. However the danger of adhesionincreases with increasing temperature of the glass piece. Moreover theevaporation of volatile components from the glass piece and thus thecontamination of the shaping tool is reduced by the method according tothe invention.

[0014] The apparatus for changing a boundary surface tension of a liquidglass piece includes an electrically conducting base on which the liquidglass piece rests and electrical contacting means connectableelectrically to the liquid glass piece to apply an electrical potentialto the liquid glass piece. The electrical conducting base is formed forheating the liquid glass piece to control the temperature of the glasspiece during application of the voltage.

[0015] The contacting means preferably includes a wire connectedelectrically to a contact position of the liquid glass piece.

[0016] Preferred embodiments of the apparatus according to the inventioninclude a source for producing an electrical potential, which is a D.C.voltage source, producing a D.C. voltage between 0.1 to 100 V.

[0017] Alternatively the source for producing the electrical potentialproduces an asymmetric alternating voltage with a D.C. voltage componentof from 0.1 to 100 V.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0018] The objects, features and advantages of the invention will now beillustrated in more detail with the aid of the following description ofthe preferred embodiments, with reference to the accompanying figures inwhich:

[0019]FIG. 1a is a schematic side view of a solid glass piece and anelectrically conducting base;

[0020]FIG. 1b is a schematic side view of a liquid glass piece and anelectrically conducting base;

[0021]FIG. 2a is a diagrammatic side view of the arrangement shown inFIG. 1b prior to applying a voltage to the liquid glass piece at anelectrical contact position;

[0022]FIG. 2b is a diagrammatic side view of the arrangement shown inFIG. 2a after applying the voltage to the liquid glass piece;

[0023]FIG. 3 is a diagrammatic view of the liquid glass piece on ashaping tool without application of a voltage;

[0024]FIG. 4 is a diagrammatic view of the liquid glass piece on theshaping tool when the base has a negative potential and the electricalcontact position has a positive potential;

[0025]FIG. 5 is a diagrammatic view of the liquid glass piece on theshaping tool when the base has a positive potential and the electricalcontact position has a negative potential;

[0026]FIG. 6 is a diagrammatic view of a liquid glass piece resting on asupporting surface with a wetting angle shown;

[0027]FIG. 7 is a diagrammatic side view of a liquid glass piece on asupporting surface showing right and left wetting angles; and

[0028]FIG. 8 is a graphical illustration showing the behavior of thewetting angles when the applied voltage applied varies with time.

DETAILED DESCRIPTION OF THE INVENTION

[0029]FIG. 1 a shows a solid glass piece 1, which is heated on anelectrically conducting base 2. It is also possible to put a heatedliquid glass piece 1 on the electrically conducting base 2.

[0030] The free surface 11 of the liquid glass piece and theelectrically conducting base 2 are electrically connected with the wire4 via the electrical contact position 3. While the glass piece has agenerally spherical shape without application of a voltage as shown inFIG. 2a, the boundary surface voltage changes because of the migrationof mobile ions in the glass network when voltage is applied to theliquid glass piece. As a result the liquid glass piece takes a differentshape.

[0031]FIG. 3 shows a liquid glass piece 1 on a structured shapingsurface 7. The peaks 8 of the shaping surface 7 penetrate the glasspiece 1 to a middle depth. No electrical potential is applied to theliquid glass piece 1.

[0032] The structured shaping surface 7 shown in FIG. 4 is on anelectrically conducting base 2 in contrast to the arrangement in FIG. 3.It is connected by means of an electrical conductor 9 with a currentsource 10. The electrically conducting base 2 is connected with thenegative pole 6 of the current source 10 in the arrangement shown inFIG. 4. The wire 4 connected to the contact position 3 is connected withthe positive pole 5. The peaks 8 present on the structured shapingsurface 7 penetrate deeper in the liquid glass piece 1 than in thearrangement without the applied voltage because of the changed boundarysurface tension.

[0033]FIG. 5 similarly shows a glass piece 1 on a structured shapingsurface 7 with an electrical connection to the free surface 11 at thecontact position 3 made by means of the wire 4. In FIG. 5 the negativepole 6 of the current source 10 is connected to the contact position 3and the positive pole 5 is connected to the electrically conducting base2. This has the consequence that the glass piece scarcely sinks into thestructured shaping surface 7 at all in comparison to the arrangementshown in FIGS. 3 and 4. Thus the glass piece is hardly structured at allwith the arrangement shown in FIG. 5.

[0034] The boundary surface tensions at the three phase boundariesbetween the solid, liquid and gas on the liquid glass piece 1 resting onthe electrically conducting base 2 are illustrated vectorially with theaid of FIG. 6. A boundary surface tension σ_(sg) resulting from thetwo-phase boundary between the solid and the gas acts outwardly from theliquid glass piece starting from an origin 12. A boundary surfacetension σ_(sl) resulting from the two-phase boundary between the solidand the liquid acts in the opposite direction. A boundary surfacetension σ_(lg) resulting from the two-phase boundary between the liquidand the gas acts tangentially to the free surface 11 from the origin 12.The boundary surface tension σ_(sl) and the boundary surface tensionσ_(lg) thus span a contact angle θ, whose value can be calculated by theYoung equation:

σ_(sg)=σ_(sl)+σ_(gl)*cos θ.

[0035]FIG. 7 shows a liquid glass piece 1 resting on an electricallyconducting base 2, which is contacted with an upper metal sheet 13instead of a wire 4. The contact position 3 comprises a comparativelylarge contacting surface region. The two-phase boundary surface tensionsσ_(lgl) and σ_(lgr) tangential to the free surface 11 on the left andright of the liquid glass piece 1 at the three-phase boundary betweensolid, liquid and gas are diagrammatically shown in FIG. 7.

[0036] Experimental results from a group of experiments are shown indiagrammatic form in FIG. 8. The graphical illustration in FIG. 8 showsthe variation of the applied voltage and the right and left wettingangles θ_(r) and θ_(l) with time. The applied voltage on the uppersurface of the metal plate 13 is read off the right ordinate, thewetting angle values off the left ordinate and the time off theabscissa.

[0037] At the beginning of the experiments the applied voltage was first0 V, the right and left wetting angles θ_(r) and θ_(l) are between 80°and 90°. Subsequently the voltage on the upper metal sheet 13 changes to−1 V and is maintained for about 400 sec. During this time both wettingangles θ_(r) and θ_(l) increase about 15 and/or 100.

[0038] Next the voltage is switched to 0 V and maintained for about 400sec. During this time the wetting angles θ_(r) and θ_(l) remain at theirpreviously attained values.

[0039] When the applied voltage is +1 V on the upper metal sheet bothright and left wetting angles θ_(r) and θ_(l) drop. The previously moresteeply increasing wetting angle θ_(l) also now drops more strongly.After 500 sec with the voltage at +1 V the voltage is switched off againfor about 600 sec. In this time interval the wetting angles θ_(r) andθ_(l) remain at an approximately constant value. Then when the voltageof −1 V is applied to the upper metal sheet 13 the wetting angles againincrease to the same extent.

[0040] The disclosure in German Patent Application 102 02 766.8-45 ofJan. 25, 2002 is incorporated here by reference. This German PatentApplication describes the invention described hereinabove and claimed inthe claims appended hereinbelow and provides the basis for a claim ofpriority for the instant invention under 35 U.S.C. 119.

[0041] While the invention has been illustrated and described asembodied in a process for changing the boundary surface tension of aliquid glass piece and apparatus for performing this process, it is notintended to be limited to the details shown, since various modificationsand changes may be made without departing in any way from the spirit ofthe present invention.

[0042] Without further analysis, the foregoing will so fully reveal thegist of the present invention that others can, by applying currentknowledge, readily adapt it for various applications without omittingfeatures that, from the standpoint of prior art, fairly constituteessential characteristics of the generic or specific aspects of thisinvention.

[0043] What is claimed is new and is set forth in the following appendedclaims.

We claim:
 1. A process for changing a boundary surface tension of aliquid glass piece, said process comprising the steps of: a) placing theliquid glass piece on an electrically conducting base; b) applying anelectrical potential to the liquid glass piece resting on theelectrically conducting base at a contact position on the liquid glasspiece; and c) heating the liquid glass piece on the electricallyconducting base.
 2. An apparatus for changing a boundary surface tensionof a liquid glass piece, said apparatus comprising an electricallyconducting base on which the liquid glass piece rests; and electricalcontacting means connectable electrically to the liquid glass piece toapply an electrical potential to the liquid glass piece; wherein theelectrical conducting base is formed for heating the liquid glass piece.3. The apparatus as defined in claim 2, wherein said electricalcontacting means comprises a wire.
 4. The apparatus as defined in claim2, further comprising a source for producing said electrical potentialand wherein said electrical potential comprises a D.C. voltage.
 5. Theapparatus as defined in claim 2, wherein said electrical potential is aD.C. voltage between 0.1 to 100 V.
 6. The apparatus as defined in claim2, wherein said electrical potential is an asymmetric alternatingvoltage with a D.C. voltage component of from 0.1 to 100 V.